Question

What is the best experimental procedure to use in choosing an eluting solvent for column chromatography?

Answer 1

Selection of solvents requires a balancing act between solvent and compound polarities. For most separations, the solvent should be less polar than the compounds. The compounds must also be soluble in the solvent so they are not permanently adsorbed. The elutropic series (order of polarity) for silica gel and alumina is as follows: hexane. petroleum ether etroleum ether is a generic name given to a mixture of pentanes, hexanes and heptanes. It does not contain any ethers of the R-O-R' structure.) < carbon tetrachloride < toluene < dichloromethane < chloroform < diethyl ether < ethyl acetate < acetone < propanol < ethanol < methanol < acetic acid < water.

In simple separations a single solvent (or solvent mixture) is used throughout the process. In complex separations, a series of increasingly polar solvents is used. By starting with a non-polar solvent, most of the compoundsshould remain adsorbed at the top of the column. Small systematic increases in solvent polarity will ideally elute these components one at a time.

A large increase in polarity may cause all of the components to elute at once, as well as cause other problems with the column packing. Consequently, small polarity changes are accomplished by careful use of mixed solvents. For example, pure hexane may be used as the first solvent. This may be followed by a mixture of 90% hexane - 10% dichloromethane. Successive mixtures containing 20%, 50%, 80%, and 100% dichloromethane would complete the transition in polarity.